This invention is directed to processes for the preparation of crosslinked resins, and more specifically to processes for preparing crosslinked styrene butadiene copolymers by free radical suspension polymerization. Thus, in one embodiment of the present invention there is provided a process for the preparation of styrene/butadiene/divinylbenzene ternary copolymers, or other crosslinked polymers by suspension free radical polymerization processes, which terpolymers are obtained in high purity in an economical manner, and are useful for the incorporation into toner compositions. Additionally, the toner compositions formulated with the resins prepared in accordance with the process of the present invention possess enhanced elasticity, and substantial toughness characteristics, that is for example they do not degrade upon impaction from carrier particles. Developer compositions can be formulated by admixing the aforementioned toner compositions with carrier particles, and these developer compositions can be incorporated into electrophotographic imaging and printing processes wherein there is obtained images of excellent resolution and substantially no background deposits; and wherein the selection of release fluids and the accompanying costly apparatuses associated therewith are avoided.
Toner and developer compositions with styrene butadiene resins are well known. There are illustrated in U.S. Pat. No. 4,558,108, the disclosure of which is totally incorporated herein by reference, processes for preparing suspension polymerized styrene butadiene resins, which resins are useful as resin particles in toner composition. Additionally, in U.S. Pat. No. 4,469,770 there are illustrated toner compositions containing plasticized styrene butadiene resin particles, which particles are prepared by emulsion polymerization methods. Further, toner compositions with certain fusible crosslinked binder polymers are illustrated in U.S. Reissue Pat. No. 31,072, see column 7 for example.
In U.S. Pat. No. 4,507,378 there is illustrated a process for the preparation of toner compositions for electrophotography which involves polymerizing an aqueous suspension of a monomer such as styrene in the presence of a dispersant which includes a phosphate compound. According to the disclosure of this patent, once polymerization occurs the dispersant is removed by adding a dilute acid and a final product is obtained subsequent to rinsing with water. Additionally, in U.S. Pat. No. 4,155,956 there is disclosed a copolymer process wherein styrene and butadiene rubber are dissolved and heated in the presence of a plasticizer and a peroxide initiating component, followed by flushing the aforementioned mixture with nitrogen gas. Thereafter, the prepolymer solution is heated in an aqueous medium with tricalcium phosphate, disubstituted sodium alkyl sulfonates, calcium carbonate, and peroxide polymerization initiators. The graft copolymers of styrene and synthetic rubber results by prepolymerizing the rubber with styrene in bulk; thus, a preformed polymer is selected as contrasted to the present invention wherein there is obtained a ternary copolymer that is crosslinked by free radical polymerization techniques. Furthermore, in U.S. Pat. No. 4,659,641 there is illustrated a polymerization process for toner resin compositions, reference Example I, wherein a styrene n-butyl methacrylate copolymer with 2 percent wax is dispersed in water. A suspending agent, tricalcium phosphate, and a surfactant in a monomer solution are added to the initial mixture with a polymerization initiator such as benzyl peroxide. After polymerization occurs, nitric acid is added to remove the tricalcium phosphate and the final product is filtered, washed, and dried.
In U.S. Pat. No. 4,125,696 there is illustrated a polymerization process with ditertiary butyl diperoxy carbonate as a finishing catalyst. Specifically, in the free radical process described in this patent a vinyl monomer is polymerized at distinct high temperature stages in the presence of two free radical initiators; however, this patent appears to be silent with respect to the preparation of crosslinked materials, and also is concerned with bulk polymerizations. Additionally, in U.S. Pat. No. 3,627,708 there is illustrated a process for the formulation of macroporous crosslinked copolymers, which are formed by a free radical type polymerization of a mixture of monomers in water, which may include styrene, ethyl vinyl benzene, and divinylbenzene. Although this patent refers to suspension polymerization emulsion polymerization was believed to be intended as, for example, a surfactant solubilizer component selected at concentrations above the micelle concentration. Polymerization occurs within the aforementioned micelles. In the present invention, process suspension polymerization is accomplished in one embodiment, for example by the utilization of minor amounts of surfactant, 1 part surfactant to about 1,700 parts of monomer by weight, below the critical micelle concentration, thereby avoiding the formation of a micelle structure. Accordingly, with the process of the present invention, in one embodiment polymerization occurs directly in the monomer beads surrounded, for example, by tricalcium phosphate or other similar materials. Further, with the process of the present invention crosslinked products of high purity result. Moreover, in the '708, there is selected an inert polymer-insoluble and monomer phobic or monomer insoluble poreforming precipitant. These precipitants are normally not soluble within the monomer mixture, but are solubilized therein through the further addition of a micelle-forming compound that acts as a solubilizer for the precipitant and is itself soluble within the monomer mixture.
Although the above mentioned processes for preparing polymer particles useful in toner compositions are suitable for their intended purposes in most instances, there remains a need for other processes wherein there can be obtained products of high purity, that is for example 99 percent or greater. Also, there remains a need for the preparation of crosslinked copolymers of high purity with enhanced elasticity, that is the ability of the product of the present invention to stretch when a stress is applied to it, and the ability of the product material to return to its original form once the stress has been removed; enhanced toughness, acceptable minimum fuse temperatures of less than or equal to about 320.degree. F., in some instances excellent (high) blocking temperature of about 125.degree. F., and acceptable micronization rates of about 175 to about 200.
There is also a need for processes that enable the preparation of crosslinked ternary copolymers by suspension free radical polymerizations, and wherein the resulting product can be selected as resin particles in the formulation of toner compositions. Furthermore, there remains a need for an economical process for the preparation of crosslinked styrene butadiene divinylbenzene ternary polymers by a suspension free radical polymerization wherein the resulting product can be obtained economically and which process is readily scalable. Additionally, there is a need for processes for the preparation of crosslinked styrene butadiene divinylbenzene ternary copolymers by suspension free radical polymerizations, which polymers can be selected for the incorporation into toner compositions that are useful for the development of images in electrophotographic imaging apparatuses wherein offset preventing fluids such as silicone oils are avoided.